This invention relates to a method for qualifying joints and contacts of an electric circuit during its production process and at the end of the production process. The invention further relates to a method for localizing a weak or bad contacting joint or contact in the electric circuit.
Methods of spot welding are known in which measuring the thermoelectric voltage between the electrode and the work-piece is the basis of spot weld qualification.
U.S. Pat. No. 5,399,827 discloses a method and apparatus for determining the temperature of a spot-welded joint and a method for assessing the quality of a spot-welded joint. After completion of the welding operation, the welding current is disconnected and the welding electrode with the work-piece, which are composed of different metals, act as a thermocouple. A temperature curve versus time is determined through the measured thermoelectric voltage. Quality of the spot weld is assessed from the course of the temperature versus time.
U.S. Pat. No. 5,721,415 describes a method and apparatus for ascertaining an evaluation time interval for assessing the quality of a spot weld based on a variation in temperature in the evaluation time interval after the spot welding. The temperature course of a good quality spot weld is approximately linear in some regions and in a middle range has a slight upward bulge that can be ascribed to a delay in the temperature drop caused by a heat of solidification at the spot weld.
Both methods described in the patents above use the heat created by the joining operation, that is the spot welding, which induces thermoelectric voltage. This voltage is generated between the work-piece and the welding electrode.
There is a particular need for a method to qualify several joints and contacts of an electric circuit after their manufacturing process, and to detect also the fault of joints, the manufacture of which does not create significant heat, such as crimping and bolting as well as to detect the fault of switches or connectors. The qualification has to be fast and reliable. Furthermore, there is a need for a method to localize the weak or bad contacting joint or contact in a defective electric circuit.